This invention relates to a process for reducing corrosion in a system in which aromatic hydrocarbons are separated from a mixture with aliphatic hydrocarbons. In particular, it relates to extracting the aromatic hydrocarbons from the mixture with an extracting solvent, stripping the extracted aromatic hydrocarbons from the extracting solvent with steam, condensing the steam to form water, separating the water from the aromatic hydrocarbons, passing the separated water through a basic anion exchange resin, and heating the water to produce the steam.
Cuts of cracked crude oil are mixtures of aromatic and aliphatic hydrocarbons. The aromatic hydrocarbons are used as solvents and chemical reactants while the aliphatic hydrocarbons are used as solvents or reactants and as olefin furnace feed. Separation of the aromatic hydrocarbons from the aliphatic hydrocarbons can be accomplished by extracting the aromatic hydrocarbons using an extracting solvent such as tetraethylene glycol. The aromatic hydrocarbons are then stripped from the extracting solvent with steam. The steam is condensed and the resulting water is separated from the aromatic hydrocarbons. To reduce pollution and maximize production, the water streams and extracting solvent streams are recycled. Gradually, these streams become more acidic due to the formation of acids, particularly acetic acid and formic acid, which corrode the system. Corrosion within the system acts as a catalyst for further acid formation, resulting in a non-linear accelerating rate of corrosion and the eventual destruction of the system.
To counteract this acidity, low concentrations (ppm) of caustic (an aqueous solution of NaOH) can be added to the extracting solvent. Caustic removes the acidity by forming water, but it also forms salts, such as sodium formate and sodium acetate. Furthermore, caustic and its salts can cause foaming in the system. The extracting solvent must then be separated from the salts by distillation. However, at the distillation temperature required to distill the extracting solvent (about 240.degree. C. for tetraethylene glycol) some of the extracting solvent decomposes, resulting in a loss of extracting solvent over time and contamination of the system with the decomposition products. This in turn causes the extracting power of the solvent to gradually deteriorate.
Another method of reducing acidity in this system is suggested in U.S. Pat. No. 4,919,816, where some or all of the water used to wash the aromatic product is passed through an anion exchange resin.
Still another approach is disclosed in U.S. Pat. No. 2,878,182, where the extracting solvent is passed through an anion exchange column.